| Abstract |
Rapidly cooled flow pathways, commonly referred to as short circuits, can pose significant problems in geothermal systems, decreasing production temperatures and reducing plant energy production and efficiency. The 2006 MIT report on the future of geothermal energy notes, for example, that one fracture in the Soultz system takes 70% of the total fluid flow through that reservoir and “this channeling, if left uncontrolled, will effectively reduce the useful recovered thermal energy of the entire reservoir.” Clearly a method is needed to reduce flow through short-circuit pathways in which cooling fronts have reached, or nearly reached, the production well. Temperature dependent ‘plugging’ agents have been proposed as one means of selectively reducing the permeability of prematurely cooled fracture flow paths. In the proposed method, compounds that are soluble or unstable at reservoir temperature, but solid and stable at slightly cooler temperature, or more viscous at lower temperature, would be injected into the cooled flow paths to reduce their permeability and thereby redistribute flow to more favorable flow paths. Several simulation studies have demonstrated how the emplaced material could effectively improve flow distribution and restore reservoir productivity. The heretofore unexplored problem with injection of temperature-dependent ‘plugging’ agents, however, is that it is difficult to emplace the material into cold flow paths without also doing so in the equally cooled upstream portion of the still ‘hot’ flow paths. In this study, we examine delivery methods that might be used to emplace such permeability reducing agents, and some of the difficulties involved in different approaches. We focus on delivery methods that leverage differences in the temperature – travel-time histories between pathways, as we believe such methods offer the most effective means of selectively altering permeability. To demonstrate some of the complexities of selective permeability modification in fractures, we use a numerical model heat and mass transfer in a simple fracture system that models disparate cooling of two fractures, and the adjacent media. We conclude with a summary of the difficulties that must be overcome to make this method, or any such method, practical for industrial application. |